The agents lie back on the rocks, waiting in silence as a fog bank rolls in, shrouding the mountains in mist.

Illegal immigrants who make it this far inside California have beaten the front line of border defenses. The only things between them and the freeways are these mountains -- and the mobile unit's Team Gold. Its members are responsible for flushing out smugglers and migrants from some of the border's most rugged terrain.

These scrub-covered peaks in the backcountry east of San Diego are becoming the illegal entry point of last resort as beefed-up enforcement in urban areas and Arizona pushes migrant routes toward more rugged terrain.

Since the agency's San Diego area sector launched the unit four years ago, its 50 agents have apprehended more than 20,000 migrants and 347 suspected smugglers. The unit, divided into three teams -- Gold, Blue and Green -- is the only one of its kind in the agency.

Few squads work harder for each arrest.

They chase smugglers along narrow trails clinging to cliffs and stage ambushes inside caves or behind rocks the size of Hummers. They crawl through vegetation-choked paths so narrow and claustrophobic they call them Hobbit trails.

The helicopter squad is dropped where it's needed along a 50-mile stretch of border.

The mountain ranges east of San Diego extend to the Imperial Valley and rise to 4,000 feet, towering over Mexico like a giant rock wall. "You'd think it would be a deterrent, but it's not," said Agent Brandon Longaker, standing on a hill overlooking the Mexican town of Tecate.

These natural barriers are cut with countless canyons, ravines and trails that snake into California. The few dirt roads are often rutted and poorly maintained, leaving much of the area out of reach of most vehicles.

As a result, members of the squad are airlifted into small clearings, the helicopters sometimes making hair- raising descents into narrow canyons. For the squad, grueling hikes deep into the mountains often follow. They work in a moonscape where unusual boulders inspire nicknames such as "Dinosaur Eggs" and "Lizard Rock."

On the starlit night of the Black Hawk landing, the agents of Team Gold walk down a wide, well-marked path that receives so much migrant traffic the agents call it a superhighway trail.

About an hour after the agents take their positions, the silence is broken by scraping noises -- feet on dirt, lots of them. Six people pass below McPartland's position. He scurries quietly down the hill and slips behind them.

Up ahead of the group, Agents Sal Selga and Wes Stephens jump from behind some bushes and block their way. One man in the front tries to bolt, knocking a woman down as he flees. The woman screams. Stephens tackles the man.

The rest of the bedraggled group gives up without a fight. In low voices, the agents identify themselves in Spanish as migra -- Border Patrol -- and order the people to be quiet to keep from alerting other migrants in the area.

Carmen Lopez, a chubby 45-year-old woman from Mexico City, says they had been hiking for two days. They had run out of food and most of their water.

Her friend from Colima, a younger woman with curly hair, has been walking on a twisted ankle, her shin bloody from a fall.

Lopez says she was trying to get to Los Angeles where she planned to work as a hairstylist.

Having made it so far into California, Lopez says, she thought she was home free. Then the agents jumped out of the darkness. "They scared me to death. I thought they were bandits," she says. "I didn't know there was Border Patrol way out here."

She says she would never try to cross again. But the agents doubt that. Sometimes, they later said, it seems like they shut down a trail only to see others slowly forming through the thick vegetation. On their flights over the mountains, they see new trails widening through the scrub, lacing ever upward.

Still, Selga said, "We do what we can. We're not going to solve the immigration problem by ourselves. But if we're not out there catching them, who will?"

The squad is heavy on former athletes, adrenaline junkies and type-A personalities who prefer adventure to long days staring at the border fence.